1. Field of the Invention
The present invention concerns a silver halide photographic emulsion with a high silver iodide content. It particularly relates to a silver halide photographic emulsion which has a high silver iodide content and whose sensitivity is improved by controlled chemical sensitization. Furthermore, it relates to a heat-developable photosensitive material which contains photosensitive silver halide with a high silver iodide content and whose performance is improved by a new chemical sensitization method.
2. Description of the Related Art
In recent years, improvements have been increasingly demanded of silver halide photographic materials in terms of high sensitivity, good storability, processability in developing, gradation, graininess and sharpness. Silver halide emulsions are usually subjected to chemical sensitization by using various types of chemical substances in order to obtain desired levels of sensitivity and gradation. Specific methods, adopted alone or in combination, include chalcogen sensitization such as sulfur sensitization, selenium sensitization or tellurium sensitization; noble metal sensitization, using a noble metal such as gold; and reduction sensitization, either using a reducing agent or taking advantage of an atmosphere conducive to reduction. In marked contrast to the many methods of chemical sensitization reported for silver iodobromide or silver chloride emulsions with a silver iodide content as low as 40%, instances of effective chemical sensitization for photographic emulsions with a high level of silver iodide, a silver iodide content of 41% to 100%, have so far been extremely rare.
For example, JP-A No. 48-87825 discloses a reduction reaction, that is, reduction sensitization applied until the formation of silver halide particles has been completed, and also discloses that a preferred silver halide for applying the sensitization is silver chloride, silver bromide or a mixture thereof, which optionally may also contain a small amount of silver iodide of up to 10 mol %. However, with regard to emulsions of high silver iodide with a content of 41 mol % or higher, there have so far been no findings at all on the advantages and disadvantages of timing for applying reduction sensitization or on securing a superior reduction capability sensitization.
Further, for example, the Journal of Photographic Science vol. 28, p 163 written by R. L. Jenkins and G. C. Farnell (published in 1980) discloses sulfur sensitization conducted with the use of sodium thiosulfate after the completion of the formation of silver iodide emulsion particles. Further, the Journal of Photographic Science vol. 22, p 174 written by D. H. Junker, E. Klein and E. Moisar (published in 1974) discloses gold sulfur sensitization applied on the insides of silver bromide emulsion particles to form latent images in the particles, so-called, internal latent type particles. However, in the case of emulsions of high silver iodide with a silver iodide content of 41 mol % to 100%, there has up to now no knowledge as to what advantages and disadvantages there would be when chalcogen sensitization or gold sensitization is applied to the insides of particles.
On the other hand, in the fields of medicine and printing production, from the viewpoints of environmental protection and space saving, there have been strong demands for dry photographic processing. In the fields described above, digitalization has made progress, and there has been a rapid popularization of systems for inputting image information into computers; storing and optionally processing the information, outputting it to photosensitive materials by means of communications at necessary sites with the use of laser image setters or laser imagers; and then developing the image information on the spot. Photosensitive materials are required to record by laser exposure at a high level of illuminance and to form clear black images having a high resolution and sharpness. As regards the digital imaging recording materials, various kinds of hard copy systems utilizing pigments and dyes, such as ink jet printers or electrophotography, have been distributed in markets as general image forming systems. However, these recording materials are not yet satisfactory in terms of the image qualities (sharpness, graininess, gradation and tone) that determine diagnostic performance, as in images in the medical field, nor are they satisfactory in terms of recording speed (sensitivity), and they have thus still not yet reached a level capable of substituting for existing medical silver salt films used in wet development.
As for dry image recording materials, thermal image forming systems are known (for example, in U.S. Pat. Nos. 3,152,904 and 3,457,075, and “Thermally Processed Silver Systems”, written by D. H. Klosterboer (Imaging Processes and Materials) Neblette, 8th edition, edited by Sturge, V. Walworth, and A. Shepp, Chapter 9, page 279, 1989). Specifically, heat-developable photosensitive materials have a photosensitive layer containing a catalytically active amount of photocatalyst (for example, silver halide), a reducing agent, a reducible silver salt (for example, organic silver salt) and, optionally, a toning agent for controlling the tone of silver, all dispersed in a binder matrix. The heat-developable photosensitive material, when heated to a high temperature (for example, 80° C. or higher) after imagewise exposure, forms black silver images by an oxidation/reduction reaction between a silver halide, or a reducible silver salt (functioning as an oxidizer), and a reducing agent. The oxidation/reduction reaction is promoted by a catalytic effect of latent images of the silver halide formed by exposure. Accordingly, black silver images are formed in an exposed region (for example, refer to U.S. Pat. No. 2,910,377 and JP-B No. 43-4924. Moreover, the Fuji Medical Dry Imager FM-DPL has been put on the market as a medical image forming system using the heat-developable photosensitive material.
Since the image forming system utilizing an organic silver salt described above has no fixing step, a significant problem has been the storing of images after development process, and in particular a deterioration in the quality of print out, particularly after exposure to light. Methods of utilizing silver iodide formed by conversion of organic silver salts as a means for improving print out have been disclosed (for example, refer to U.S. Pat. No. 6,143,488, EP-No. 0922995). However, sufficient sensitivity could not be obtained by means of the method of converting organic silver salts with iodine described therein, and it proved difficult to come up with a practical system. While photosensitive materials utilizing silver iodide are also described in several patent documents (for example, refer to WO 97-48014, WO97-48015, U.S. Pat. No. 6,165,705, JP-A No. 8-197345 and Japanese Patent No. 2785129), none of them has yet reached a sufficient standard in terms of sensitivity and prevention of fogging, or can stand up to practical application as laser exposure photosensitive materials.
As a means of improving sensitivity of silver iodide photographic emulsions, sensitization has been known in technical documents, sensitization, for example, by using halogen receptors such as sodium nitrite, pyrogarol or hydroquinone, by dipping into an aqueous solution of silver nitrate or by sulfur sensitization at pAg 7.5 (for example, refer to Photographic Science and Engineering, vol. 18(5), p 475, by P. B. Gilman, (published in 1974), Photographic Science and Engineering, vol. 18(5), p 475, by W. L. Gardner, (published in 1974), Photographic Science and Engineering, vol. 5, p 216, by T. H. James, (published in 1961). However, as shown in the examples the sensitizing effect of the halogen receptors is extremely low and conspicuously inadequate for the purpose of the heat-developable photosensitive materials of the type used by the present invention. Accordingly, there have been strong demands for the development of a technique capable of raising appreciably the level of sensitivity in heat-developable photosensitive materials with a high silver iodide content.
In the case of silver halide photosensitive materials for liquid development (for example, refer to U.S. Pat. Nos. 5,747,235, 5,747,236, 6,054,260 and 5,994,051), a method of sensitization is known involving the use of a compound forming two electrons with one photon.
However, in the silver halide photosensitive material for liquid development, silver halide is generally reduced by a developing chemical (reducing agent) contained in a processing solution to form silver images, or alternatively color images are formed by utilizing an oxidant by-product of a developer, and in either case a basic reaction is the reduction of the silver halide by a developer. In contrast, in the heat-developable photosensitive material, silver halide merely forms latent images by exposure, and silver halide itself is not reduced by the reducing agent but on the other hand what are reduced are silver ions supplied from a non-photosensitive organic silver salt. As regards reducing agents, while an ionic reducing agent such as hydroquinone or p-phenylenediamines is used in the case of liquid development, in the case of a heat-developable photosensitive material, a hindered phenol derivative known as a radical reaction agent is generally used.
As described above, for on the one hand the photosensitive material for liquid development and for on the other hand the heat-developable photosensitive material, the mechanisms of a developing reaction (reduction reaction) are quite different, and the compounds used also belong to entirely different categories. Accordingly, it cannot be always assumed that a compound which is effective in the case of liquid development will automatically also be effective in the case of a heat-developable photosensitive material. Also, in the case of the compounds described in the group of US patents mentioned above, when they are applied to the heat-developable photosensitive materials, there is no guarantee at all that identical effects would be obtained and equally it cannot be predicted that completely different effects would be obtained. Furthermore, it is inconceivable that they could be applied to heat-developable photosensitive materials that use emulsions of high silver iodide content and it has been impossible to foresee the effects of doing so.
Accordingly, photographic emulsions of a high silver iodide content with a high degree of sensitivity are required. Further, outstanding heat-developable photosensitive materials of high sensitivity with a low propensity for fogging are also necessary.